The main application of adhesion promoters is in so-called plastisols. These are paste-like dispersions of a powdered, fine-particle polymerization product in a plasticizer. The polymers are primarily polyvinyl chloride, polyvinyl chloride copolymers, and polyalkylene methacrylates.
Plastisols are used, among other things, for thermal-insulation and soundproofing layers, glueing and sealing of materials, and protection against corrosion and damage from flying stones, the latter primarily in automobile assembly through application on a variety of metal surfaces. Plastisols have a relatively weak adhesive strength, which in no way meets today's stringent requirements. For example, if after application of the protective layer, the adhesion at certain locations is insufficient or gives way over time due to mechanical stress, the underlying metal can quickly corrode. An adhesive layer can be provided between substrates such as metal surfaces and the protective layer, but this involves a number of disadvantages. It is more advantageous, however, to add an adhesion promoter to the plastisol, enhancing the adhesion of the plastisol mixture itself.
However, the adhesion promoter, integrated in the plastisol before the latter is applied to the surface to be coated, should not only improve adhesion but also contribute to improved plastisol workability and moreover have sufficient thermal stability to prevent undesired discolorations when the coating is baked. Furthermore, the adhesion promoter should enable the use of minimum baking temperatures and times. It should also be efficient enough that these and other objectives can be attained with minimum quantities.
Most adhesion promoters available on the market and also described in the very extensive patent literature and professional journals are manufactured on the basis of polymerized unsaturated fatty acids and polyamines. Condensation of the polymerized fatty acids and the amines results in so-called polyaminoamides with alternating polymer structure.
There has been no lack of attempts to improve the manufacture of the polyaminoamides used for this specific application, including modifying the ratio of monomers, dimers, and trimers constituting the unsaturated fatty acids, or otherwise modifying the chemical structure, such as by using other amines.
In this regard, European patent application 171 850, for example, describes adhesion promoters based on polymerized fatty acids of a specific composition and 1.0 to 2.2 equivalents of a heterocyclic amine, and European patent application 378 205 teaches that mixtures of polyalkylene polyamines and N-aminoalkylpiperazine can be used to construct the polyaminoamides.
The unsaturated fatty acids are natural products whose composition is subject to fluctuations. Furthermore, manufacture of the polyaminoamides is not always controllable.
One of the disadvantages of such products is that they have sometimes lacked the required stability with respect to thermal stress. Uncontrollable reactions often occur during polymerization or condensation, such that it is difficult, if not impossible, to construct an effective polymer with a specific molecular weight. Adhesion and color stability also leave something to be desired at times.
There have also been attempts to construct fully synthetic adhesion promoters, since they are constructed from compounds with a precise chemical definition, rather than from nonuniform natural products, and they are also reproducible with reasonable certainty. Canadian patent application 2,035,538, for example, describes adhesion promoters involving copolymers of aliphatically/olefinically unsaturated compounds and an anhydride. The aliphatically unsaturated compounds can also be .alpha.-olefins, and the anhydride can be maleic anhydride. The adhesion promoters described in this cited patent application are not completely satisfactory with respect to improving adhesive effectiveness compared to the adhesion promoters based on polyamines and polymerized fatty acids. Although numerous adhesion promoters are currently known, there is still a need for adhesion promoters with improved characteristics such as adhesion agency, efficiency, thermal stability, color stability, workability, and advantageous production methods.